Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a photosensitive coating deposited on the substrate, the substrate and the photosensitive coating having opposite surface properties (such as hydrophilic vs. oleophilic, and oleophobic vs. oleophilic). The photosensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation, optionally with further post-exposure overall treatment. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate.
Traditionally the plate is exposed with an actinic light (usually an ultraviolet light from a lamp) through a separate photomask having predetermined image pattern that is placed between the light source and the plate. Laser sources have been increasingly used to imagewise expose a printing plate that is sensitized to a corresponding laser, allowing the elimination of photomask, reducing material, equipment and labor cost.
Among the laser imagable plates, infrared laser sensitive plates, also called thermosensitive plates or thermal plates, are most attractive because they often can be handled and processed under white light.
U.S. Pat. No. 5,491,046 describes a lithographic plate having on a substrate a thermosensitive layer comprising a resole resin, a novolac resin, a haloalkyl substituted s-triazine, and an infrared absorber. U.S. Pat. No. 6,117,610 describes a lithographic plate having on a substrate an infrared imaging composition comprising a non-basic infrared radiation absorbing material and a phenolic resin that is either mixed or reacted with an o-diazonaphthoquinone derivative. These plates can be imagewise exposed with an infrared laser and then developed with an aqueous alkaline developer to form positive imaging.
U.S. Pat. No. 6,153,356 describes a lithographic plate having on a substrate a photopolymerizable composition comprising an ethylenically unsaturated compound, a cyanine dye, and a photopolymerization initiator. U.S. Pat. No. 6,232,038 describes a lithographic plate having on a substrate a photosensitive composition comprising an ethylenically unsaturated double bond-containing compound, a sensitizing dye, and a photopolymerization initiator. These plates can be imagewise exposed with an infrared laser to cause hardening in the exposed areas and then developed with an aqueous alkaline developer to form negative imaging.
While the plates developed with aqueous alkaline developer are capable of superior lithographic quality, the use of aqueous alkaline developer has two major drawbacks. First, the alkaline developer is caustic liquid that is a health hazard and requires special handling. Second, alkaline developer is very hard to maintain for the target alkaline strength in the plate processor due to the constant reaction of the alkaline compound with carbon dioxide from the air.
Thermosensitive lithographic plates not requiring an alkaline developer have been proposed in the literature. For example, U.S. Pat. Nos. 5,674,658 and 5,677,106 describe lithographic plates comprising on a porous hydrophilic substrate an oleophilic radiation-absorbing heat-sensitive layer that is capable of bonding to the porous substrate through thermal flow upon exposure to an infrared laser. The non-exposed areas can be removed by contacting with ink or by lamination and peel development step. While such plates do not require alkaline developer, they suffer from poor press durability.
Therefore, there is a desire for a thermosensitive plate that does not require an alkaline developer and has excellent press performance, and a method of developing such a plate.